Skip Navigation

Cancer Overview

Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce and are often shaped differently from healthy cells. They do not function properly and can spread to many areas of the body. These cells can form tumors, which are clusters of cells that are capable of growing and dividing uncontrollably.

Doctors who treat cancer are called oncologists. Oncology is the medical specialty that focuses on the diagnosis and treatment of cancer.

What do the terms benign and malignant mean?

Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors tend to grow slowly and do not spread to other parts of the body. Malignant tumors can grow rapidly, invade and destroy nearby normal tissues and spread throughout the body.

What do the terms locally invasive and metastatic mean?

Cancer is malignant because it can be locally invasive and metastatic.

  • Locally invasive: The tumor can invade tissues surrounding it by sending out "fingers" of cancerous cells into the normal tissue.
  • Metastatic: The tumor can send cancerous cells into other tissues in the body, even if they are distant from the original tumor.

What are primary tumors?

The original tumor is called the primary tumor. Its cells can break off and travel through the body and begin forming additional tumors in other organs. These new tumors are called secondary tumors. The cancerous cells travel through the blood (circulatory system) to form secondary tumors. They may also travel through the lymphatic system, which is a series of small vessels that collect waste from cells and carry it into larger vessels, and finally into lymph nodes. Lymph fluid eventually drains into the bloodstream.

How is each cancer type named?

Cancer is named after the part of the body where it originated. When cancer spreads, it keeps this same name. For example, if kidney cancer spreads to the lungs, it is still kidney cancer, not lung cancer. The cancer in the lung would be an example of a secondary tumor. Staging is the process of determining whether cancer has spread and, if so, how far. More than one system is used for staging cancer, and the definition of each stage depends on the type of cancer.

What are the different types of cancer?

Cancer is not just one disease but rather a group of diseases, all of which cause cells in the body to change and grow out of control. Cancers are classified by the kind of fluid or tissue they come from or by the place in the body they first develop. Some cancers are of mixed types. The following broad categories indicate the tissue and blood classifications of cancer.

  • A carcinoma is a cancer found in epithelial tissue, which covers or lines surfaces of organs, glands or body structures. For example, a cancer of the lining of the stomach is called a carcinoma. Many carcinomas affect organs or glands that are involved with secretion, such as breasts that produce milk. Carcinomas account for 80%-90% of all cancer cases.
  • sarcoma is a malignant tumor growing from connective tissues, such as cartilage, fat, muscle, tendons and bones. The most common sarcoma, a tumor on the bone, usually occurs in young adults. Examples of sarcoma include osteosarcoma (bone) and chondrosarcoma (cartilage).
  • Lymphoma refers to cancer that begins in the nodes or glands of the lymphatic system. The lymphatic system produces white blood cells and cleans body fluids. Some lymphomas start in lymph tissue in organs like the brain or stomach. There are 2 types of lymphoma: Hodgkin lymphoma and non-Hodgkin lymphoma.
  • Leukemia, also known as blood cancer, is a cancer of the bone marrow that keeps the marrow from producing normal red and white blood cells and platelets. White blood cells are needed to fight infection. Red blood cells are needed to prevent anemia. Platelets keep the body from easily bruising and bleeding. Types of leukemia include acute myelogenous leukemia, chronic myelogenous leukemia, acute lymphocytic leukemia and chronic lymphocytic leukemia. The terms myelogenous and lymphocytic indicate the type of cells that are involved.
  • Myeloma grows in the plasma cells of bone marrow. In some cases, the myeloma cells collect in one bone and form a single tumor, called a plasmacytoma. In other cases, the myeloma cells collect in many bones, forming many bone tumors. This is called multiple myeloma.

What causes cancer?

There is not a single cause for cancer. Scientists believe it results from the interaction of many different factors. The factors may be genetic, environmental or lifestyle traits of the individual.

What are the risk factors for cancer?

Some cancers are associated with certain risk factors. A risk factor is anything that may increase a person's chance of developing a disease. A risk factor does not necessarily cause the disease, but it may make the body less resistant to it. People who have an increased risk of developing cancer can help to protect themselves by scheduling regular screenings and checkups with their physician and avoiding certain risk factors.

  • Lifestyle factors such as smoking, a high-fat diet and exposure to sunlight are risk factors for some adult cancers. Most children with cancer, however, are too young to have been exposed to these lifestyle factors for any extended time. Their exposure as children may put them at risk for cancer later in life. 
  • Family history, inheritance and genetics play an important role in some adult and childhood cancers. It is possible for cancer of varying types to be present more than once in a family. Some gene alterations are inherited. But this does not necessarily mean that the person will develop cancer. It indicates that the chance of developing cancer increases. In such cases, it is unknown whether the disease is caused by a genetic mutation, other factors or simply coincidence.
  • Exposures to certain viruses have been linked to an increased risk of developing cancer. Such viruses include the human papillomavirus (HPV) and human immunodeficiency virus (HIV, the virus that causes acquired immune deficiency syndrome, or AIDS). It's possible that the virus alters a cell in some way. That cell then reproduces an altered cell and, eventually, these alterations become a cancer cell that produces more cancer cells. Cancer is not contagious and a person cannot contract cancer from another person who has the disease.
  • Environmental exposures have been linked to some cancers. For example, people who have certain jobs seem to have an increased risk of some cancers. Painters, farmers, construction workers and people in the chemical industry may be frequently exposed to certain chemicals that can cause cancer. Other exposures may occur in the home or elsewhere. Exposure to radon (a radioactive gas) in homes can put people at risk.

How do genes affect cancer growth?

The discovery of certain types of genes that contribute to cancer has been an extremely important development for cancer research. Nearly all cancers have some type of genetic alteration. A small percentage (5%-10%) of these alterations are inherited, while the rest occur by chance or from environmental exposures over time. There are 3 main types of genes that can affect cell growth, and are altered (mutated) in certain types of cancers:

  • Oncogenes
    These genes regulate the normal growth of cells, causing them to grow. Scientists commonly describe oncogenes as similar to a cancer "switch" that most people have in their bodies. What flips the switch to make these oncogenes suddenly allow abnormal cancer cells to grow is unknown.
  • Tumor suppressor genes
    These genes are able to recognize abnormal growth and reproduction of cancer cells. And they can interrupt their reproduction until the defect is corrected. If the tumor suppressor genes are mutated and don't function properly, tumor growth may occur.
  • Mismatch-repair genes
    These genes help recognize errors when DNA is copied to make a new cell. If the DNA does not match perfectly, these genes repair the mismatch and correct the error. If these genes are not working properly, however, errors in DNA can be transmitted to new cells, causing them to be damaged.

Usually, the number of cells in our body tissues is tightly controlled so new cells are made for normal growth and development, and to replace dying cells. Genetic alterations tip the balance in favor of excessive cell growth, causing cancer.

How do childhood and adult cancers differ?

Diagnosis, treatment and prognosis for childhood cancers are different than for adult cancers. The main differences are the survival rate and the cause of the cancer. The 5-year survival rate for childhood cancer is about 80%, while in adult cancers the 5-year survival rate is 68%. This difference may be because childhood cancer is more responsive to therapy and a child can tolerate more aggressive therapy.

Childhood cancers often occur or begin in the stem cells, which are simple cells capable of producing other types of specialized cells that the body needs. A cell change that occurs by chance or mutation is usually what causes childhood cancer. In adults, the type of cell that becomes cancerous is usually an epithelial cell, which line the body cavity and surfaces of organs, glands or body structures, and cover the body surface. Cancer in adults usually occurs from environmental exposures to these cells over time. Adult cancers are sometimes referred to as "acquired" for this reason.

Related links